This invention relates to high intensity arc discharge lamps and more particularly to high intensity arc discharge metal halide lamps having high efficacy.
Due to the ever-increasing need for energy conserving lighting systems that are used for interior and exterior lighting, lamps with increasing lamp efficacy are being developed for general lighting applications. Thus, for instance, arc discharge metal halide lamps are being more and more widely used for interior and exterior lighting. Such lamps are well known and include a light-transmissive arc discharge chamber sealed about an enclosed pair of spaced apart electrodes, and typically further contain suitable active materials such as an inert starting gas and one or more ionizable metals or metal halides in specified molar ratios, or both. They can be relatively low power lamps operated in standard alternating current light sockets at the usual 120 Volts rms potential with a ballast circuit, either magnetic or electronic, to provide a starting voltage and current limiting during subsequent operation.
These lamps typically have a ceramic material arc discharge chamber that usually contains quantities of metal halides such as cerium iodide (CeI3) and sodium iodide (NaI), or praseodymium iodide (PrI3) and NaI, or other rare earth halides such as dysprosium iodide (DyI3), holmium iodide (HoI3), and thulium iodide (TmI3), and thallium iodide (T1I), as well as mercury to provide an adequate voltage drop or loading between the electrodes, and the inert starting gas. Keeping the lamp operating voltage below 110V rms results in relatively safe operation of the lamp and its ceramic arc discharge chamber. Such lamps can have an efficacy as high as 105 LPW at 250 W with a Color Rendering Index (CRI or Ra) higher than 60, with Correlated Color Temperature (CCT) between 3000 K and 6000 K at 250 W.
Of course, to further save electric energy in lighting by using more efficient lamps, high intensity arc discharge metal halide lamps with even higher lamp efficacies are needed and lamps which maintain well their luminous output over the operational duration thereof. The lamp efficacy is affected by the shape of the arc discharge chamber. If the ratio between the distance separating the electrodes in the chamber to the diameter of the chamber is too small, the relative abundance of Na between the arc and the chamber walls leads to a lot of absorption of generated light radiation by such Na due to its absorption lines near the peak values of visible light. On the other hand, if the ratio between the distance separating the electrodes in the chamber to the diameter of the chamber is too great such as being greater than five, initiating an arc discharge in the arc discharge chamber is difficult because of the relatively large breakdown distance between the electrodes. In addition, such lamps perform relatively poorly when oriented vertically during operation in exhibiting severe colors segregation as the different buoyancies of the lamp content constituents cause them to segregate themselves from one another to a considerable degree along the arc length.
Another problem with such metal halide lamps is the gradual reduction of the light output over the lamp operational duration due to the reduced light transmission through the walls of the arc discharge chamber. The darkening of the chamber wall is mainly attributable to sputtering of the electrode tungsten material during the starting of light emission in the chamber of the lamp, and to the evaporation of the electrode tungsten material in that chamber during subsequent lamp operation. In many instances, such coating of the arc discharge chamber walls by tungsten not only results in poor lamp output light lumen value maintenance but also to the premature failure of the lamp.
That such objectionable coating of the arc discharge chamber walls does not occur more quickly and completely than it typically does is generally thought to be due to a regenerative tungsten halide transport cycle phenomenon occurring in the chamber in which the deposited tungsten metal on the wall is returned to the electrodes thereby tending to keep the chamber walls clean. In this cycle, the tungsten material deposited on the chamber walls is thought to combine there with iodine from the ionizable constituents provided in the chamber to form tungsten iodide which then evaporates from the chamber wall to thereafter impinge on the electrodes. There, the tungsten iodide disassociates there with the iodine evaporating to thereby leave the tungsten deposited on the electrodes. An efficient halogen cycle of this sort results in excellent lamp light output lumen value maintenance and a long operational duration for the lamp.
One condition known for an efficient halogen cycle is the presence of a small amount of oxygen in the discharge chamber when the lamp is being operated. Thus, a metal halide lamp has been used with oxygen dispensers containing tungsten oxide (WO2) and calcium oxide (CaO) to avoid arc discharge chamber tungsten coating and to extend lamp life. A small amount of free oxygen is released at a controlled rate into the chamber to aid in maintaining the halogen cycle. Success requires that the release of free oxygen be controlled. When too small an amount of oxygen is released, the halogen cycle will not operate as well resulting in early coating of the chamber walls. If, on the other hand, too much oxygen is released, the tungsten electrodes suffer extensive corrosion resulting in a short lamp operational duration due to electrode failure. Further alternatives include providing oxygen in the form of oxytrihalides such as niobium oxytriiodide (NbOI3) or mercury oxide (HgO) or molecular oxygen or compounds containing oxygen to the chamber constituents. Metal oxyhalides, particularly tungsten oxyhalides, such as WOI2, WO2Br2 and WOBr2, will be formed during the operation of the lamp. However, such additions add expense to the manufacture of the lamp. Thus, there is a desire for a lamp that provides good efficacy with the output light lumen value well maintained while being operable by currently used ballast circuits.